Boris Johnson has call for an "earned amnesty" for thousands of illegal immigrants living in London, claiming that the notion that they will one day be deported from the UK is "just not going to happen".The Conservative mayor of the capital is to commission a study into the feasibility of granting an amnesty for people living illegally in the city. It is expected to report in two months' time.Johnson is willing to depart from the Conservative party line in his belief that an estimated 400,000 people who have lived illegally in the capital for years should have the chance to "earn" their citizenship in order to play a full part in London life, including paying their taxes.Although he has no legal powers over immigration policy, Johnson said he wanted to "lead the debate" on the issue.

Johnson argued that a mass "programme of expulsions" would be both costly and legally difficult. A more sensible idea, he said, would be to introduce "earned amnesty", whereby after a period of about five years individuals could "show their commitment to this society and to this economy" to earn the right to stay."We want to look in detail at what the economic impact of such an earned amnesty system would be," Johnson told Channel 4 news last night."There are about 400,000 in London. That's a huge number. In principle these people have done the wrong thing: they've broken the law. In principle they should all be taken and sent back to their place of origin, that's the right thing to do ... (but) unfortunately it is just not going to happen."Johnson is expected to lobby the Labour government once the feasibility study is complete, although both Labour and the Conservatives regard the issue as politically toxic and have in the past attacked the Liberal Democrats for suggesting it.Johnson first floated proposals for an amnesty for long-standing illegal immigrants during his mayoral campaign, prompting the Conservative leader, David Cameron, to comment that amnesties "just store up" the need for further amnesties in the future.A Tory spokesman said last night: "We will have to agree to differ on this. One-off amnesties have been tried elsewhere and the evidence is that they do not work, but lead to more."The Conservative group within the London assembly are also opposed to such a scheme, believing it to be unworkable without stronger border controls to dissuade people from entering the country illegally with the aim of being granted legal status under future amnesties.Johnson stressed he was not seeking to set up incentives for illegal immigration, but to regularise and decriminalise those already in the system."You don't want to create moral hazard, but I think you should have a system whereby people who have been here for a long time can earn a way out of the mess they're in," he said.Johnson's spokesman conceded that the mayor "cannot change the laws on immigration", but added: "It is his job as mayor to speak out for people in London on issues where he has relevant things to say.

"He is not committed to lobbying the government on this but he has committed to explore the issue fully."The launch of the review coincided with the day City Hall held its first-ever London-wide citizenship ceremony.A resident from every one of London's 33 boroughs attended the ceremony and was presented with a certificate and commemorative gift by the mayor and the chair of the London assembly, Jennette Arnold.